A laser beam emitted from a laser oscillator is excellent in its directivity and light-focusing capability so that it is easy to focus the laser beam into a minute spot by lenses and/or mirrors and is possible to obtain high energy density. For this reason, the laser oscillator is utilized widely in the field of machining such as cutting, drilling, welding or thermal processing in recent years.
The laser oscillator is generally constituted of one partially reflecting mirror (hereinafter referred to as an “output mirror”) placed on its side from which a laser beam is emitted, and of other mirror(s) being one or a plurality of totally reflecting mirror(s), to thereby cause oscillation on the principle that a laser beam is amplified for emission by multiply reflecting between the mirrors. When the laser oscillator is used for long hours, absorption of the laser beam will occur in the mirrors placed in the laser oscillator because of degradation of mirror coating layer or degradation of mirror material itself, causing nonuniform temperature distribution within the mirrors. The nonuniform temperature distribution results in nonuniform refractive index distribution, causing changes in laser-beam properties or reduction in beam power of the laser beam; therefore, in order to maintain machining quality, a regular cleanup and/or replacement of the mirrors placed in the laser oscillator, or so-called maintenance is required. In particular, because the output mirror allows transmission of a laser beam through it, the output mirror is, in comparison with other totally reflecting mirrors, likely to cause absorption of the laser beam and also difficult to be cooled from its back face, allowing its cooling only from its lateral side, so that nonuniform temperature distribution is likely to occur within the mirror.
For this reason, in order to maintain the machining quality, maintenance of the output mirror placed in the laser oscillator is conventionally performed based on a criterion time, that is empirically obtained as a guide, without measuring the mirror degradation condition.